1. Field of the Invention
The present invention relates to an image processing method and apparatus for performing the process of inhibiting noise, wrinkle, and spot components on a photographic image to enhance the picture quality, and to a program for carrying out that method.
2. Description of the Related Art
Skin-beautifying processes have hitherto been performed for the purpose of inhibiting or eliminating wrinkles and spots from a photographic image containing a person's face (hereinafter referred to simply as an image). For example, low-pass filters that are usually used for eliminating noise can be applied. However, while low-pass filters can inhibit wrinkle, spot, and noise components from an image, they will degrade an edge portion contained in an image signal and blur the entire image.
An ε-filter (ε-separation nonlinear digital filter), which is designed to separate and inhibit a high-frequency noise component of low amplitude contained in an image by generating use of the fact that many of the wrinkle and spot components exist as signals of low amplitude in the high-frequency component of an image, is also applied to eliminate wrinkles and spots (Arakawa, et al., “Face Image Processing by a Vector(ε)-Filter, and Elimination of Wrinkle Components,” Drafts in the National Meeting of the Academic Society for Electronic Information Communication, March 1998, D-11-43, p. 143). Since the ε-filter has the property of flattening only a change in the level of low amplitude contained in an image signal, an image processed by the ε-filter preserves edges having a sharp change in the level of the amplitude and hardly loses the entire sharpness.
The ε-filter is basically constructed to subtract from an original image signal a value obtained by applying a nonlinear function to a change of quantity in the level of the amplitude of the signal. This nonlinear function outputs a value of 0 when the amplitude of a signal is greater than a predetermined threshold value (ε0). That is, when the ε-filter is employed, the output of the nonlinear function is 0 at a part in an image that has an amplitude greater than the aforementioned threshold value. In a processed image, the original signal of a part having an amplitude greater than the aforementioned threshold value is preserved. On the other hand, in a part whose amplitude is the aforementioned threshold value or less, the signal value of that processed part is a value obtained by subtracting the output of the nonlinear function (where the absolute value is greater than 0) from the original signal value. In this manner, at parts in the processed image which contain not so-called noise but wrinkles and spots exhibiting a change in light and darkness of low amplitude, the change in light and shade is smoothed. As a result, edges whose amplitude is high can be preserved while generating wrinkles and spots indistinct.
Japanese Unexamined Patent Publication No. 2001-118064 discloses a method for preserving skin grain and texture while eliminating wrinkles and spots. In this method, wrinkle and spot components are extracted by a nonlinear function having a large threshold value (ε0), and grain and texture components are extracted by another nonlinear function having a small threshold value (ε0). The extracted wrinkle and spot components are subtracted from the original signal, and the extracted grain and texture components are added to the original signal. In this manner, skin grain and texture are preserved while eliminating wrinkles and spots.
In addition, a variety of techniques have been proposed for extracting signals of different frequency bands from an image. For instance, U.S. Pat. No. 5,991,457 discloses a method for generating a plurality of blurredly masked images different in frequency response characteristics from one another, based on an original image and also generating a plurality of band-limited images respectively representing signals of different frequency bands of the original image, based on the original image and blurredly masked images, or based on the blurredly masked images. U.S. Pat. No. 5,991,457 discloses a method for efficiently generating a blurredly masked image by reducing the amount of calculation required in generating the blurredly masked image.
However, many of the wrinkle and spot components exist in high-frequency bands, but they exist over the entire range from high-frequency bands to low-frequency bands. The aforementioned skin-beautifying process method utilizing the ε-filter cannot completely eliminate wrinkle and spot components, because it extracts the wrinkle and spot components only in a single frequency band and subtracts the extracted components from the original image. In the aforementioned method for extracting wrinkle and spot components at a single frequency band, if the effect of eliminating wrinkles and spots are to be improved, filtering must be enhanced at the single frequency band, that is, the aforementioned threshold value (ε0) for extracting wrinkles and spots must be increased. However, this can easily introduce artifacts into a processed image and debase the picture quality.